1. Field of the Invention
The present invention relates to a milling apparatus for production of semiconductor devices and the like. More specifically, the present invention relates to a milling apparatus for milling a substrate material by projection of rare gas ions generated by a thermoelectron emission type ionization mechanism.
2. Related Background Art
Milling processes are conventionally employed for preparation of electron microscope specimens, fine patterning of an object such as optical devices, magnetic bubble memories, and the like. The milling process is wanted to be improved for treatment at a lower temperature of a treatment object.
A production process which employs the milling treatment is explained by taking as an example a process for production of a magnetic domain wall displacement type magnetooptical disk disclosed in Japanese Patent Application Laid-Open No. 2001-167479. In the process for producing magnetooptical disk disclosed in this patent disclosure, lands and grooves which serve respectively as a track are formed alternately on an information plane with interposition of tilt zones having a face nonparallel to the information track plane, then a recording film is formed on the information track plane of a substrate of a recording medium, and the recording film deposited on the tilt zones is removed selectively.
This removal of the deposited film from the tilt zone prevents diffusion of the heat generated by a projected laser beam or by a heating means for magnetic domain displacement to the adjacent tracks on recording or reading, preventing the adverse effect of the heat on the recorded information on the adjacent track, namely cross-erasing/writing. The magnetooptical disks produced by such a process can be improved by narrowing the interval between the tracks, or increasing the track density, to increase the recording capacity without causing the cross-erasing/writing.
In this process for production of the magnetooptical disk, a milling treatment can be employed for removing the recording film from the tilt zones. For the magnetooptical disks, less temperature-resistant substrates are suitably used, such as polycarbonate plates, and glass plates covered with a resin layer. On the other hand, the milling is generally conducted by sputter-etching by projecting high-energy ions onto a substrate. The projected ions give a high energy to the substrate. Since this milling energy will raise the temperature of the substrate, a milling method is wanted which enables the treatment at a lower temperature to avoid the adverse effect on the substrate.
In a conventional milling process, ions are projected onto a substrate from an electron impact type ion gun placed in opposition thereto, generally with water cooling of the holder for holding the substrate to prevent excessive temperature rise of the substrate. Japanese Patent Application Laid-Open No. 1-301870 discloses a milling apparatus which uses an ion source similar to the one employed in a usual milling process but is provided with a substrate-holder equipped with a substrate-cooling mechanism of a water-cooling type to prevent the temperature rise of the substrate, and conducts milling by moving the substrate-holder between a treatment chamber and an annexed chamber.
However, in widely employed conventional milling processes, the radiant heat of the filament inside the ion-gun is introduced directly onto the substrate, which is one cause of temperature rise of the substrate. In conventional milling processes, an ion beam-neutralizing electron-emitting source is provided near the substrate to prevent the charging-up of the substrate, the radiation from the filament thereof being another cause of the temperature rise of the substrate. Further, in these processes, for directing the emitted gas ions to the substrate, an electrostatic force of a lead-in electrode is utilized with application of a constant voltage. The voltage applied to this electrode is controlled to be relatively high to direct the ions onto the substrate effectively. The higher lead-in voltage increases the energy of the projected ions to cause the temperature rise of the substrate.
Owing to the above causes, in the conventional milling processes, the temperature of the substrate rises inevitably to some extent even though the substrate is cooled indirectly by water-cooling of the substrate holder. When a substrate used has significantly low heat-resistance as in the aforementioned magnetooptical disks, the temperature rise cannot be kept sufficiently low not to cause deformation of the substrate or a like adverse effect.
The milling apparatus disclosed in Japanese Patent Application Laid-Open No. 1-301870 is improved in the setting and delivering means for the substrate for simplification, by using a substrate-cooling device similar to the one of conventional milling process. In the above apparatus, regarding the substrate cooling function, the substrate set on the substrate holder under an atmospheric pressure is milled as it is without transfer to another holder, whereby the substrate is pressed to the substrate holder to improve somewhat the cooling effect without remarkable improvement.